1. Field of the Invention
The present invention relates to a pixel cell array for silicon LC light valves, and in particular, to a pixel cell array having a planar alignment layer of uniform thickness formed over the active pixel electrodes.
2. Description of the Related Art
Liquid crystal displays (LCDs) are becoming increasingly prevalent in high-density projection display devices. These display devices typically include a light source which passes light through a light valve.
One of the methods for producing colors in a liquid crystal display is to sequentially project light having a wavelength corresponding to a primary color onto a single light valve. Color sequential light valves create a spectrum of color within the range of the human perception by switching between a set of discrete primary colors. Typically, red, green, and blue are the primary tri-stimulus colors used to create the remaining colors of the spectrum.
Specifically, during projection of each primary color, the light intensity is modulated such that combination of the intensities of the primary colors in sequence produces the desired color. The frequency of switching between the primary wavelengths by the light valve should be sufficiently rapid to render discrete primary states indistinguishable to the human eye. Two factors dictate the minimum frequency necessary for switching.
The first factor is the ability of the human eye to detect the discrete primary colors (e.g., red, green, blue). At slower than ideal switching speeds, the human eye will detect a flicker and the primaries may not blend.
The second factor determining the frequency of switching is the video refresh rate. During display of video images, the individual frames must be refreshed at frequencies undetectable to the human eye.
The net frequency of switching demanded by the combination of sequential color blending and video refreshing is beyond the capabilities of light valves that utilize thick ( greater than 1 xcexcm) liquid crystal (LC) transducers. However, thin ( less than 1 xcexcm) liquid crystal transducers have been successfully fabricated. These thin LC transducers demonstrate adequate color sequential blending at video refresh rates. One example of such a thin LC transducer pixel cell structure is disclosed in U.S. Pat. No. 5,706,067 to Colgan et al.
In general, the conventional thin LC transducer pixel cells possess enhanced responsiveness due to the decreased volume of liquid crystal material between the top and bottom plates. A smaller volume enables the liquid crystal to shift orientation more quickly and in response to a lower applied voltage.
FIG. 1 shows a cross-sectional view of a adjacent pixels of a conventional array for a silicon LC light valve. Array portion 100 includes molecules 102 of liquid crystal (LC) sandwiched between a top plate 104 and a bottom plate 106. Top plate 104 includes a translucent substrate 108, typically glass or plastic, having an underside coating of a transparent, electrically conducting material that forms passive pixel electrode 109.
Bottom plate 106 is formed by the reflective surfaces of the pixel electrodes 112a and 112b. Active electrodes 112a and 112b, and passive electrode 109, are coated with first and second alignment layers 111b and 111a respectively. Alignment layers 111a-b (typically composed of polyimide) provide an anchoring surface for ends 102a of the LC material 102 interposed between the active pixel electrode and the passive pixel electrode. Alignment layers 111a-b are typically scored in order to ensure that LC material 102 is aligned in a particular direction in response to an applied electric field.
The bottom plate 106 is formed by the active reflective metal pixel electrodes 112a and 112b of adjacent pixels 110a and 110b, respectively. Pixel electrodes 112a and 112b are separated and electrically isolated by trenches 118 in inter-pixel regions 119.
Pixel electrodes 112a and 112b lie on top of an upper intermetal dielectric layer 128 that forms a component of interconnect scheme 104. Interconnect 104 overlies capacitor structures 120a and 120b formed within underlying silicon substrate 105. Capacitor structure 120a includes a dielectric layer 162 formed over a double diffused drain (DDD) region 160 created within silicon substrate 105. Capacitor structure 120a further includes a polysilicon contact component 164 formed over dielectric layer 162.
Storage capacitors 120a and 120b are in electrical communication with pixel electrodes 112a and 112b, respectively, through metal via plugs 140, middle interconnect metallization layer 124, and lower interconnect metallization layer 122. Storage capacitors 120a and 120b are controlled by MOS switching transistors 142a and 142b, respectively. MOS switching transistors 142a and 142b are also formed in underlying silicon substrate 105, and are electrically isolated from adjacent semiconducting devices by trench isolation structures 144.
FIGS. 2A-2F illustrate cross-sectional views of the process for forming the conventional thin LC transducer pixel cell shown in FIG. 1.
FIG. 2A illustrates the starting point for the conventional process. Starting structure 200 is created by forming an upper intermetal dielectric layer 128 over a lower interconnect metallization layer (not shown). Portions of upper intermetal dielectric layer 128 corresponding to the center of future pixel regions are then etched to stop on the lower interconnect metallization layer, forming vias. These vias are then filled with electrically conducting material to create via plugs 140, and then the electrically conducting material is removed outside of the vias.
Next, reflective metal electrode layer 112 is formed over upper intermetal dielectric layer 128 and the tops of via plugs 140. Photoresist mask 150 is then patterned over reflective metal electrode layer 112 to expose inter-pixel regions 119.
FIG. 2B shows etching of reflective metal electrode layer 112 in unmasked inter-pixel regions 119 selective to upper intermetal dielectric layer 128, defining discrete reflective pixel electrodes 112a and 112b separated by trench 118. Photoresist mask 150 is then removed.
At this point in the process flow, the chip upon which the pixel array is being formed is transferred from a conventional silicon processing facility to a one that specializes in the handling of liquid crystal material. Prior to introduction of liquid crystal material to the pixel array, a surface must be formed over the active and passive electrodes that permits uniform alignment of liquid crystal material within the cell.
Accordingly, FIG. 2C shows the flowing of a quantity of alignment material 111 over the entire surface, including on top of active reflective pixel electrodes 112a and 112b, and within trench 118. Alignment material 111 is typically formed from polyamic acid, water, and a solvent which is spun onto the wafer in liquid form.
FIG. 2D shows the curing of alignment material 111, during which solvent is removed and alignment material 111 shrinks and hardens to form alignment layer 111b conforming to raised active pixel electrodes 112a and 112b. As shown in FIG. 2D, once alignment layer 111b has solidified, the thickness of this layer is non-uniform over the surface of active pixel electrodes. Alignment layer 111b includes a thick portion 111c at the center of the active pixel electrodes 112a and 112b. 
At this point in the process flow, alignment layer 111b is scored by a rubbing wheel, which traverses the surface of the pixel cell and gouges alignment layer 111b in a uniform direction.
FIG. 2E shows completion of assembly of the pixel cell by disposing LC material 102 over the active pixel electrodes 112a and 112b, and then sealing top plate 104 including passive pixel electrode 109 and first alignment layer 111a over LC material 102.
FIG. 2F shows the effect of application of a voltage bias to active pixel electrodes 112a and 112b through via plugs 140. Application of a voltage bias in this manner creates electrical field 150 across LC material 102 between active pixel electrodes 112a and 112b, and passive pixel electrode 109. The presence of electric field 150 causes LC material 102 to align in a uniform direction anchored at either end by first and second alignment layers 111a and 111b. 
The conventional fabrication process described above in FIGS. 2A-2F is adequate to produce functional thin LC transducer pixel cells. However, the conventional process flow suffers from a serious disadvantage in the formation of an alignment layer having low planarity and non-uniform thickness over the active pixel electrode.
As described above, alignment of the LC material under the influence of an applied electric field is a critical light value attribute. Alignment of the twisted nematic LC dictates polarization of incident light which will pass through the LC. In the context of a complete system, this alignment of the LC material defines either the black or white extreme of the light valve""s gray scale.
Liquid crystal material overlying the pixel electrode has the propensity to align and/or tilt with any topology present on the surface of the pixel cell. The increased thickness of the second alignment layer at the center of the active pixel electrode creates topography on the pixel cell surface. As a result, non-uniformity in alignment of LC material due to uneven thickness in the alignment layer will translate into a poorly constructed display.
Therefore, there is a need in the art for a process for forming a thin LC transducer pixel cell that creates a planar alignment layer of uniform thickness on the surface of the active pixel electrode.
The present invention relates to a pixel cell array and to a process for fabricating a pixel cell array that creates a planar alignment layer of uniform thickness over the active pixel electrode.
Specifically, the process in accordance with a first embodiment of the present invention proposes forming the alignment layer in two stages. A first, small quantity of alignment material having low viscosity is applied in liquid form over the array surface. The low viscosity and small volume of this first quantity of alignment material allows it to settle within the trenches in inter-pixel regions, leaving the reflective array surface substantially free of alignment material. The first quantity of alignment material is then cured to form a hardened lower alignment material within the trenches.
Next, a second, larger quantity of higher viscosity alignment material is flowed over the reflective array surface and the lower alignment layer. Because the lower alignment layer previously formed within the trenches substantially reduces topology offered by the surface of the pixel array, curing of the second quantity of alignment material yields a substantially planar alignment surface of uniform thickness over the active pixel electrodes.
A process flow for forming a pixel cell in accordance with one embodiment of the present invention comprises the steps of forming a plurality of discrete raised active pixel electrodes over an intermetal dielectric layer, the active pixel electrodes separated by trenches, the active pixel electrodes and trenches creating a topography. A first quantity of alignment material in liquid form is flowed over the active pixel electrodes such that the first quantity of alignment material settles within the trenches. The first quantity of alignment material is cured to form a lower alignment layer within the trenches, the lower alignment layer reducing the topography offered by the trenches and the discrete pixel electrodes. A second quantity of alignment material is flowed in liquid form over the discrete pixel electrodes and the lower alignment layer. The second quantity of alignment material is cured to form a planar surface alignment layer of uniform thickness over the discrete pixel electrodes and the lower alignment layer.
An array of pixel cells for a silicon light valve comprises a plurality of raised reflective active pixel electrodes formed on top of an intermetal dielectric layer and electrically isolated from each other by trenches, the plurality of reflective pixel electrodes and the trenches creating a topography. An alignment material is positioned within the trenches and reduces the topography offered by the plurality of active pixel electrodes and the trenches. A surface alignment layer is formed over the plurality of pixel electrodes and the alignment material, the surface alignment layer substantially planar and of substantially constant thickness across a width of the active pixel electrodes.
The features and advantages of the present invention will be understood upon consideration of the following detailed description of the invention and the accompanying drawings